The present invention relates generally to a process and apparatus of purifying a liquid, and, in particular, relates to a process and apparatus for purifying N.sub.2 O.sub.4 that is used as a propellant.
It has been known since the late 1960's that N.sub.2 O.sub.4 propellant shipped and stored in mild steel containers may contain deleterious amounts of corrosion products (i.e., particulate and colloidal iron nitrate adduct contaminants) which can cause filter clogging or flow decay and leakage. This can result in the reduction of oxidizer flow to the combustion chamber in thrusters which in turn may have major effects upon delicate orbital correction maneuvers. Therefore, it is essential that these contaminants in N.sub.2 O.sub.4 be eliminated or reduced prior to loading the propellant onto space launch vehicles.
A prior process for reducing iron contaminants to around 0.5 ppm requires chilling the propellant to below 40.degree. F. The lower temperature reduces the solubility of the contaminants in N.sub.2 O.sub.4. This is followed by filtration of the chilled propellant through a fine membrane filter (5 micron Teflon). Although this process has achieved its goal of reducing contaminants, there are several derawbacks: (1) Chilling and filtering large quantities of propellant are costly and very time consuming operations; (2) Due to the sometimes gelatinous and colloidal nature of the iron contaminants, filtration through fine membrane filters may not always be effective for removing iron contaminants from N.sub.2 O.sub.4 ; and (3) The effectiveness of removing gelatinous material by filtration is known to be highly pressure dependent.
Another prior process is by distillation. However, an additional step required in the prior laboratory distillation process was a complete degassing prior to the start of distillation by freezing and evacuating. This removed NOCl and volatile iron contaminants. This step is also extremely slow and costly for large quantities of propellants.
Filtration of N.sub.2 O.sub.4 at ambient temperatures through a 10 micron Teflon filter by itself showed no reduction in iron concentration.
The above drawbacks have motivated a search for a process and apparatus to purify N.sub.2 O.sub.4.